docs: def observer_update(self, message):
          """Called when update from observer is received."""
          # Demultiplex observer update into multiple messages.
          for action in ('added', 'changed', 'removed'):
              for item in message[action]:
                  self.send_json(
                      {
                          'msg': action,
                          'observer': message['observer'],
                          'primary_key': message['primary_key'],
                          'order': item['order'],
                          'item': item['data'],
                      }
                  )